Thin-walled tubes of a zirconium-based alloy, for example a zirconium-tin alloy, are normally used as cladding tubes for the fuel for nuclear reactors. Such tubes may be internally lined with a layer of copper to increase their resistance to stress corrosion induced by fission products. For neutron economy it is necessary for this copper lining layer to be thin. One problem in connection with such internally lined cladding tubes is that, during the operation of the nuclear reactor, copper from the lining layer diffuses into the cladding tube, so that after a time the copper lining layer loses its ability to protect the tube. One known method for preventing the diffusion of copper into a zirconium-based alloy tube is to arrange a layer of zirconium dioxide between the internal surface of the tube and the copper lining layer. The zirconium dioxide layer, which thus serves as a barrier against the diffusion of copper into the zirconium-based alloy, may be applied as a coating on the tube by oxidizing the tube before the copper lining layer is applied thereon. The zirconium dioxide layer may also be produced after the copper lining layer has been applied to the tube by bringing the tube with its copper lining layer, at elevated temperature, into contact with water vapor or some other substance which has the ability to oxidize zirconium but does not have the ability to oxidize copper.
It is known to activate the surface of a tube of a zirconium-based alloy by treating it with an activating solution before the tube is provided with a coating of copper or other metal, either electrolytically or by chemical precipitation. In this connection there has been used as the activating solution an aqueous solution containing from 10 to 20 grams per liter (g/l) of ammonium bifluoride and from 0.75 to 2 g/l of sulfuric acid or an aqueous solution containing 0.2 mole/liter (7.4 g/l) of ammonium fluoride and 0.13 mole/liter (2.6 g/l) of hydrogen fluoride, respectively.
The present invention aims to provide a method of applying a layer of copper and a layer of zirconium dioxide to the internal surface of a tube of a zirconium-based alloy, wherein a markedly improved adhesion of the copper layer to the base material is achieved.